The present invention relates generally to implantable devices such as a cardiac pacemaker. More particularly, this invention relates to a programmable implantable device having a plurality of operating modes.
Improper heart function can often be remedied by the use of an implantable device such as a pacemaker. These devices generally provide an electrical pulse to a selected area of the heart that is not (in terms of timing or strength) adequately receiving its natural pulse. Under abnormal cardiac conditions, and particularly cardiac rhythm disturbances, pacemaker therapy is applied to remedy several forms of cardiac arrhythmias (rhythm disturbances) including bradycardias, AV conduction block, supraventricular tachycardias, and atrial and ventricular ectopic arrhythmias. There are essentially two types of pacemakers: single-chamber (capable of sensing and pacing in either the atrium or the ventricle only); and dual-chamber (capable of sensing and pacing in both the atrium and the ventricle). From a practical standpoint, there are essentially only two forms of single-chamber pacing: VVI (senses and paces in the ventricle) and AAI (senses and paces in the atrium). On the other hand, there are many modes of dual-chamber pacing such as VDD (paces in the ventricle only, senses in the atrium and ventricle), DVI (paces in the atrium and ventricle, and senses in the ventricle only), DDI (senses and paces in both the atrium and ventricle), and DDD (senses and paces in both the atrium and ventricle, with an inhibited and triggered response to sensing). A letter xe2x80x9cRxe2x80x9d is sometimes added to these pacemaker modes to indicate the pacemaker""s ability to provide rate-modulated (also sometimes called rate-responsive or rate-adaptive) pacing. For instance, a DDDR pacemaker is capable of adapting to the need to increase a patient""s heart rate in response to physiologic stress, even if the patient""s intrinsic SA node would not normally allow this to occur.
Modern pacemakers typically incorporate a microprocessor and one or more memory units. These pacemakers are configured to allow remote programming after implantation in the patient""s body, and offer advanced features such as sensor-control, programmable dual-chamber pacing and sensing, and rate-modulated cardiac pacing. External and noninvasive programming is accomplished using telemetry circuits which allow a clinician to communicate with and program the implanted pacemaker using an external programmer. Examples of programmable pacemaker functions include the pacing mode, rate, and pulse voltage and width. To program a pacemaker externally, signals (usually pulsed magnetic fields or radio frequency signals) are transmitted through the patient""s skin between the programmer and the pacemaker""s control unit.
Microprocessor-based pacemakers have proven to be of great practical utility, because they do not impose unneeded trauma on the heart and they provide therapy on as-needed basis. However, these devices do occasionally suffer from malfunction and error. Such malfunction or error is often of significant concern, because a person""s life may depend on the device""s proper operation. In these devices, errors could be caused by a malfunction in the hardware (the electronics) or by deterioration in the software, which might occur over time due to a memory or power failure.
To reduce the severity of such malfunctions, some pacemakers have been designed to automatically transition to a xe2x80x9cfail-safexe2x80x9d, pacing mode upon the detection of certain errors. In this mode, commonly referred to as a xe2x80x9creset mode,xe2x80x9d the programmable, xe2x80x9cnormal modexe2x80x9d parameters are effectively replaced with a fixed set of default or xe2x80x9creset modexe2x80x9d parameters. (xe2x80x9cParametersxe2x80x9d are stored values that specify the pacing therapy that is to be administered by the pacemaker.) The reset mode parameters typically include, for example, the pacing rate, the pulse width, and the pulse amplitude to be used in the reset mode. The general goal of operating in a reset mode is to eliminate the pacemaker""s dependence on the malfunctioning component (such as a corrupted or defective memory area), while continuing to apply a pacing therapy to the heart.
Although the above-described approach allows the patient to continue to receive therapy, the pacing therapy applied in the reset mode is not always adequate, particularly in patients that suffer from tachyrhythmia, bradycardia, and atrial fibrillation. In addition, the pre-set voltage provided to the heart in the reset mode can be too high for the particular patient, and may therefore interfere with the heart""s normal operation and/or unnecessarily reduce the life of the battery.
One partial solution to the above-described problem has been to provide a reset mode which preserves some of the automaticity functions of the pacemaker. (xe2x80x9cAutomaticityxe2x80x9d refers generally to the ability of a pacemaker to make logical decisions and adjust pacing therapies based on physiological variations within the patient.) One problem with this approach is that the parameters that govern reset mode operation are typically stored in a read-only memory (ROM), and thus cannot be programmed by the clinician. Thus, although the pacemaker automatically adjusts the reset mode therapy according to the patient""s symptoms, these adjustments are made according to a pre-specified pacing program, and the clinician cannot customize this program to individual patients.
The present invention overcomes the aforementioned and other problems by providing an implantable system and associated method that allow programming of the reset parameters, thereby allowing a physician to customize the reset mode in conformance with the particular therapeutic needs of a patient. The present invention thereby provides a safer, more reliable implantable system.
In accordance with one form of the invention, an implantable system includes a pulse generator, mode switching means, and external programming means. The pulse generator senses the electrical activity in the heart, recognizes certain needs for the electrical stimulation, and delivers the appropriate pulses with sufficient energy to initiate depolarization of the cardiac tissue. The mode switching means monitors the operation of the implantable system and, upon detecting a malfunction, switches the system from the normal mode to the reset mode of operation. The programming means allow a clinician to program the parameters for the normal and reset modes of operation.
To perform its functions, the pulse generator includes a microprocessor-based controller for determining the proper timing of electrical stimuli that are provided to the heart. The pulse generator further includes two or more memories where it stores program parameters for controlling the normal mode and reset mode of operation. A first programmable memory is utilized to store a set of xe2x80x9cnormal modexe2x80x9d parameters that specify a full-featured mode of operation, and a second programmable memory which stores a set of xe2x80x9creset modexe2x80x9d parameters. Both sets of parameters are initially set at the factory, and can thereafter be modified by telemetry (before and/or after implantation) by the clinician. The programmable parameters include the pacing mode, pacing rate, sensor status, pulse configuration, sensitivity level, and several others. Because the reset parameters are programmable, a fail-safe or reset mode is provided that can be customized to meet the specific therapeutic needs of a particular patient.
In response to detected errors, the switching means switch the mode of operation of the implantable system from the normal mode to the reset mode. In doing so, the normal operating mode parameters are ignored and the pulse generator becomes governed by the reset operating parameters. This switchover minimizes the probability of, or in certain circumstances completely eliminates, the use of defective components in the pacemaker. By providing the clinician with the ability to program the reset operating parameters, the present invention allows the particular therapeutic needs of the patient to be served in the event of a malfunction in the implanted pacemaker.